How to upgrade the DRZ400 Stock Mikuni BSR36 Carb to the Keihin FCR39 MX Carb.

Much has been written about the OEM DRZ400 Mikuni BSR36 carb, you can jet it, tune it, tweak it, but in the end, it’s still a CV carb with limited potential and lacklustre performance. Check here to see if your DRZ400 has the CV Carb. You have probably decide the extra HP and better throttle response of a well jetted FCR39 carb was worth the price. What follows is installing the FCR39 on to your DRZ400 which is not hard. Though some general knowledge of motorcycles and Carbs specifically is important anyone with decent mechanical ability and basic tools can perform this install.

First, you need to acquire a Keihin FCR39 MX or FCR39 carb. You can find these on eBay but be careful – there are several worn out carbs on eBay from other makes/models of bikes that can potentially cause you a headache down the road. ThumperTalk.com has a Keihin FCR39 MX kit that fits all DRZ’s having a BSR36 carb, All S and SM models, and 2007 and later E model bikes. This is probably your best bet. Below is a description of what they include.

Prepping the new Carb and bike for install

Remove and discard the plastic throttle wheel guard, it will interfere with fitment relevant to the frame, and really is not needed.

Move both upper motor mounts to the left side; this allows needed clearance for the throttle wheel
The TPS connector will not be reused with the new FCR carb, you can follow the carb end up wiring to the connector and unplug it there. Use some dielectric silicone and tape to blank off the connector, you can cut the wires there, heat shrink or tape the ends, or simple stuff the carb end of the TPS and wiring harness up under the seat, zip tie it in place and leave it. Bottom line, it is not used for this install.

Remove pilot air jet it is not used for this application

If using a vacuum operated fuel petcock like the OEM one found on the DRZ S or SM, you will need to install a vacuum nipple in the manifold. Luckily there is a spot just for this and an OEM part to install. It is P/N 13685-29F00 OEM Suzuki Vacuum Port Nipple and included in the TT Kit.

Intake adapter too short? (see bottom of page for FAQ).

DRZ400 Basic Jetting (FCR39 MX Carb)

Install the jetting required for your motor modification level and riding area. Below are four of the most common jetting setups. For many, that’s all they will need to do, for some, additional tuning will be required. Use what is here for a base.

The Fuel needle clip position is counted from the top, flat end of the needle. The needle pictured is in clip 3 position.

Install tips

No need to buy new throttle cables, the stock S cables work fine. Attach the throttle cables with the curved one on the bottom.

Hoses are routed like this

DRZ400 Jetting Accessories

In addition to the above or custom jetting, an extended adjustable fuel screw is highly recommended.
Merge Racing makes a very nice one.

Be very careful on the removal and install of the new fuel screw.. you have a small spring, washer and rubber O-ring that sits on the screw inside the body. One or more of these part sometimes remains in the body, and comes out haphazardly on the end of the old fuel screw. Very easy to lose one or more parts.

Installation FAQ

Q: The intake adapter appears to be too short. What now?

A: The intake adapter really is not too short. People say this all the time due to the physical measurement between old carb set up and FCRMX being different, the FCR setup is slightly shorter. The air boot is plenty long enough. The preferred technique is to put the carb in the boot first. Then pull it forward to the manifold. If you are working in a cold environment, take a hair dryer to the boot and try heating it up slightly until it become plyable.

Just got my Keihin FCR 39mm
For my 09′ SM. Love it but I used my own
Boots that I sized up from Home Depot
For cheap. Call me what you want but I
Rode all weekend with no gas or vacuum
Leaks. I also did not have to move motor
Mounts only because I wAs scepticle.
Rock n Roll! Bwaap bwaap